I think you should have a long talk with your psychiatrist about this. Ask them to schedule you for an hour at least to talk in depth about the risks and benefits of different meds. I would guess that the first step would be to try you on something that's reasonably safe in pregnancy, and make sure it's actually keeping you stable.
A number of meds are category C, which basically means there's no studies in humans to show if there's harm or not, there may be studies in animals pointing to possible issues, and that it's up to the doctor and patient to decide on the risks/benefits. Ambien is category C, but it seems like a lot of doctors are fine with people taking it during pregnancy, and if it will help stave off episodes, it's probably worth it.
Personally, I think I would probably stay on meds during pregnancy (not that I'm planning to try that anytime soon), because of the risks of depression in pregnancy and the fact that depression is my main issue and I'm on a drug that's category C. If being unmedicated has a reasonable chance of making you a complete wreck, either through raging mania having you do unsafe things/wrecking your finances or through horrific depression, then I think the benefits of taking meds would outweigh the risks. But it's a very personal decision, and you should talk it out thoroughly with your psychiatrist before deciding.
If you want to talk more in depth about it i would love to. I just don't want to get in anymore information online where everyone can see. I have alot of advice that helped for me. Im also a psych student so i learned alot from my schooling that i instilled into my own life.
I was diagnosed with bipolar disorder 10yrs ago. I have a 4yr old and Im now 7weeks pregnant with our second child. I personally did not take medication during either pregnancy. I do have to admit though i had a very very hard time sleeping thru my first and now my second. I did ok off my meds. I had a very strong support system though. I made sure to stay on top of everything. Talked to my husband alot about how i was feeling mentally. I did however go back on medication immediately following the birth of my son because i was so scared of developing post partum depression. I do have to admit though people think pregnant women are moody, well they obviously haven't lived with a pregnant nonmedicated bipolar woman. Lol. :)